The invention relates to an electro-optical display device comprising an electro-optical display medium between two supporting plates, means for indicating the temperature of the electro-optical medium, and means, responsive to said indication, for generating a corrected voltage dependent on said indication to compensate for changes in said temperature.
Display devices of the kind mentioned are used in, for example, portable computers, telephones, car displays, etc.
An electro-optical display device of the kind mentioned above is described in EP-A-0.642.113 (PHN 14.599). To compensate for the variation of the transmissionvoltage characteristic curve due to temperature changes in a ferro-electric display device, the display device is provided with one or more measuring elements for measuring the polarization current. The drive voltages are corrected by measuring the polarization current and comparing this to the polarization current at 50% transmission level to obtain an indication for the temperature.
Due to the fact that measured values are compared in a separate comparator inaccuracies occur e.g. because of errors in signal propagation, cross-talk and inaccuracy in the comparator.
One of the aims of the invention is to provide an electro-optical display device in which temperature sensing and further processing are simplified.
Another aim is to prevent errors due to signal propagation as much as possible.
An electro-optical display device according to the invention is characterized in that said means for indicating the temperature of the electro-optical display medium comprise a temperature sensor which provides a digitized temperature value and is arranged on one of the supporting plates.
By providing the temperature as a digitized value, it can immediately be processed, for example, in a separate microcontroller or in a microcontroller in the same integrated circuit in which the temperature sensor is realized to generate the necessary correction. Since the temperature values are digitized, errors in signal propagation due to cross-talk hardly occur. A temperature sensor providing a digitized temperature value is described in xe2x80x9cMicropower CMOS Temperature Sensor with Digital Outputxe2x80x9d by A. Bakker and J. H. Huijsing, IEEE, Journ. of Solid-State Circuits, Vol. 31, No 7, pages 933-937.
In known displays, thermistors are used as means for obtaining indications on the temperature, or diodes, in which the temperature dependency of the forward voltages on temperature are used. In these cases, analog values of (reference) voltages are obtained separately which are used for further processing to compensate for temperature changes. Separate AD-converters, are used in said compensation methods. The analogue value is converted at places different from the temperature sensing point, leading to errors in conversion due to voltage loss (in transmission), and temperature difference between the measuring spot and the place where actual conversion occurs.
To obtain an optimal thermal coupling between the electro-optical material and the temperature sensor, the temperature sensor is realized in an integrated circuit, the integrated circuit being face-down bonded to said one of the supporting plates.
The output of the temperature sensor (comprising, for example, a microprocessor and a programmable memory or a look-up table) controls (a) voltage driver(s) (a column driver or row driver or a combination). In a preferred embodiment at least the temperature sensor and the voltage driver(s) are realized on said one supporting plate by means of an integrated circuit. In a further preferred embodiment the temperature sensor and the voltage driver(s) are realized on said one supporting plate by means of thin film (transistor) technology.